Ground loops and powerpoint presentations

[WynnD]

Just finished providing sound for a two day conference. Ran into an occasional ground loop issue in one of the rooms. Expected it to be the laptop's power source so I ran an extension cable from the PA to the power strip provided. That worked for some of the laptops, but not all. In one case the laptop was running on battery power and the only other connection was the VGA cord to the built in projector. (Controls mounted in the wall and the projector would lower from the ceiling. No idea what line it was on.) I was able to minimize the issue, but would like to hear suggestions for preventing this in the future.

 

Met Reverend Billy there. What a character.

[agedhorse]

You have just discovered why the pros use isolation transformers. There are several potentila sources of ground loops and specifically through the video equipment grounds along with the computer's SMPS noisy grounds make isolation transformers the defacto standard anytime I interface to video or laptop.

[WynnD]

That was the only room that was a problem. Most of the presenters didn't use audio out of their laptops.

 

Got a suggestion on a good value unit for that?

[agedhorse]

 

That was the only room that was a problem. Most of the presenters didn't use audio out of their laptops. Got a suggestion on a good value unit for that?

 

 

PM me. I build all custom units depending on the exact application.

[RoadRanger]

Got a suggestion on a good value unit for that?

This one goes from dual RCA or 1/8" stereo to dual XLRs :

http://www.audiopile.net/products/DI_Boxes/DBRC-2A/DBRC-2A_cutsheet.shtml

[KF650SB1000]

I would believe that you have an older Dell laptop?

[agedhorse]

There are lots of problem machines, and as soon as you connect to video, it opens up a whole new world of possibilities.

[Flogger59]

Whirlwind makes the PCDI just for that purpose.

www.whirlwindusa.com

[WynnD]

In that room there were about 8 presenters each day. Everyone had their own laptop. (Macs and Pcs) Didn't even use my own. The permanent installation without a circuit marked for the projector is the cause of the problem. (How many different circuits could there possibly be?) Can different circuits in exactly the same phase also cause a ground loop? If not, then just finding another outlet in phase would help. (Not the kind of thing you want to be doing last minute.) By the way, I have some passive DI boxes, and that didn't help. This probably won't be an issue for me until the next conference in two years.

[Craig Vecchione]

 

Can different circuits in exactly the same phase also cause a ground loop? If not, then just finding another outlet in phase would help. (Not the kind of thing you want to be doing last minute.)

 

 

Yes, you can have a ground loop with two outlets on the same circuit, not just the same phase. The phase doesn't really have anything to do with a ground loop. Any difference in resistance between the grounds on those outlets is all that's needed. In theory, a ground wire running from outlet A to outlet B should have low resistance. In practice I've found 3 or more ohms in wires that are only 20 or 30 feet long, thanks to poor connections and corrosion.

[WynnD]

Thanks for all the tips. Will apply them the next time this comes up. Didn't know about wire resistance being a possible cause either. (Might just have been lucky so far.)

[agedhorse]

Because of al the possibilities, isolation transformers are the only reliable solution, hence the reason why the real pros carry them. It's an essential tool.

[Craig Vecchione]

 

Didn't know about wire resistance being a possible cause either. (Might just have been lucky so far.)

 

 

 

To be clear it's the resistance of the connections, not the wire itself. It would literally take miles of wire to reach the kind of resistance seen in the worst connections.

 

It's really not all that common, especially in locations with a building code and inspections before occupancy. If the building was wired by a halfway competent electrician the connections will stay sound for decades. But things can go wrong with even the best material and workmanship.

[karlw]

I used to keep an iso transformer in my laptop bag just for stuff like this. Learned the hard way before that... it's definitely a must.

[WynnD]

I've got 4 Rolls passive DI boxes. Either there's something wrong with them, or even their configuration wasn't going to cure this hum. (I just put an extreme dip in the frequency response at 60 htz. The only other thing going thru the PA was speech.) Now I am confused. Anyone got testing suggestions?

[RoadRanger]

Just to speculate - I use one of these:

http://www.artproaudio.com/products.asp?type=90&cat=13&id=106

Unlike a passive DI it uses 1:1 isolation transformers that give as much higher output (~ +19db?) as I believe a 50K to 600 ohm transformer (such as in the rolls) is about 9:1? Perhaps that reduces the hum by that amount over a DI? I use a standard 1/8" stereo to dual RCA patchcord from the laptop to the box.

[Craig Vecchione]

 

I've got 4 Rolls passive DI boxes. Either there's something wrong with them, or even their configuration wasn't going to cure this hum. (I just put an extreme dip in the frequency response at 60 htz. The only other thing going thru the PA was speech.) Now I am confused. Anyone got testing suggestions?

 

 

Not all DI's have transformers, and without one, isolation is difficult to impossible.

[WynnD]

Was pretty sure the Rolls had them. Will open one up and look again. (And test for continuity across it. With the ground lifted, there shouldn't be any.)

[Craig Vecchione]

 

Was pretty sure the Rolls had them. Will open one up and look again. (And test for continuity across it. With the ground lifted, there shouldn't be any.)

 

 

 

Another aspect is that the laptop is introducing noise into the signal itself. In your OP the laptop is on battery power, and its VGA output is driving an overhead projector, which is connected to AC power. The projector's VGA connection could pass hum to the laptop, which doesn't filter it from the soundcard, injecting it into the audio signal. At this point, no isolation transformer can help, nor would a ground lift, as the 60Hz hum is now part of the voice/music signal, and all you can do is what you did; try to EQ it out. My guess is that in your instance it wasn't actually a ground loop but a noisy component, in this case the overhead projector. The noise gets passed along a data line in the VGA cable, and the laptop doesn't filter it so at some point it winds up in the audio signal.

 

I've also experienced similar problems, albeit with different noise, using an iPod into a laptop. Both are running on battery, and you hear an annoying whine/buzz/hum from the iPod's hard drive as it spins up and runs.